Changing current frame without updating depsgraph
If you already modified things go ahead and create a new keyframe that overwrites the previous. Go to the dopesheet, select the newly created keyframe (by selecting the yellow diamond icon) and copy it (CtrlC). Then press CtrlZ a few times to undo the changes you had made in the pose and get your previous keyframe back. In the dopesheet move to a different frame and paste (CtrlV) the modified keyframe, now you have both keyframes.

(But the easiest solution is to press the right arrow key so you are in a different frame before modifying anything...)